The present invention relates to an apparatus and a method for digital signal processing that convert a PCM digital signal having a sampling frequency of fs (Hz) and a multi-bit as a quantization bit into a high-speed 1-bit digital signal having a sampling frequency of m (m is a positive integer ≧2)×n (n is a positive integer ≧2)×fs (Hz) and one bit as a quantization bit.
A ΔΣ modulated high-speed 1-bit audio signal has a format including a very high sampling frequency and a short data word length (for example a sampling frequency of 64 times 44.1 kHz and a data word length of 1 bit) as compared with a format of a PCM signal used in conventional digital audio (for example a sampling frequency of 44.1 kHz and a data word length of 16 bits). The ΔΣ modulated high-speed 1-bit audio signal has an advantage of a wide transmissible frequency range. Even with the 1-bit signal, the ΔΣ modulation makes it possible to obtain a wide dynamic range in an audio range of low frequencies relative to the 64x oversampling frequency. Taking advantage of this feature, the ΔΣ modulation can be applied to a recorder and data transmission for high-quality sound.
A ΔΣ modulator itself is not an especially novel technique; a ΔΣ modulator is a commonly used circuit within a conventional A/D converter or the like, because the circuit configuration is suitable for integration into an IC and the circuit can achieve a high A/D conversion precision relatively easily.
The ΔΣ modulated signal can be restored to an analog audio signal by passing the ΔΣ modulated signal through a simple analog low-pass filter.
Conventionally, when converting a PCM musical sound signal with a quantization frequency of 44.1 kHz and a quantization word length of 16 bits into a high-speed 1-bit audio signal with a quantization frequency of 2822.4 kHz (64×44.1 kHz) and a quantization word length of 1 bit, for example, only a PCM frequency range of up to 22.05 kHz is converted into a high-speed 1-bit audio signal by oversampling and ΔΣ modulation. Hence, a frequency range of the musical sound signal after the conversion is also up to 22.05 kHz, and thus a frequency range of the high-speed 1-bit audio signal of up to about 100 kHz is not used.